Tunnelexperimente an den Chevrel Supraleitern CU$_{1.8}$MO$_{6}S$$_{8}$ und PBMO$_{6}$S$_{8}$

Poppe, Ulrich

Book/Report

FZJ-2018-01414

Tunnelexperimente an den Chevrel Supraleitern CU$_{1.8}$MO$_{6}S$$_{8}$ und PBMO$_{6}$S$_{8}$

Poppe, U. (Corresponding author)FZJ*

1980
Kernforschungsanlage Jülich, Verlag Jülich

Jülich : Kernforschungsanlage Jülich, Verlag, Berichte der Kernforschungsanlage Jülich 1635, 72 p. (1980)

Please use a persistent id in citations: http://hdl.handle.net/2128/17480

Report No.: Juel-1635

Abstract: The electron-phonon couplingin ternary molybdenum sulfides was studied by electron tunneling spectroscopy. For the first time, direct measurements of the energy gap of the superconducting Chevrel-phases Cu$_{1.8}$Mo$_{6}$S$_{8}$ and PbMo$_{6}$S$_{8}$ were performed. For Cu$_{1.8}$Mo$_{6}$S$_{8}$ we used point contact tunneling diodes with oxidized aluminium- or GaAs-tips. In the case of PbMo$_{6}$S$_{8}$ we fabricated diodes by evaporating an artificial barrier and a counterelectrode on small single crystals. Extraordinarily large deviations from the BCS-behavior indicate that Cu$_{1.8}$Mo$_{6}$S$_{8}$ and PbMo$_{6}$S$_{8}$ are strong coupling superconductors showing a 2$\Delta$$_{o}$/kBTc ratio of -4.1 and -4.7, respectively. Furthermore we observed phonon induced structure in the quasiparticle density of states by measuring the second derivitave of the current-voltage characteristic. Measurements of the phonondensity of states of several Chevrel-phases with inelastic neutron scattering showed that within a sbnple molecular crystal model it is possible to group the phonons into low lying external modes and high frequency internal modes of the M06SS units. In contradiction to former suggestions that only special phonon modes are responsible for superconductivity in Chevrel-phases, we find a coupling to all the external modes in Cu$_{1.8}$Mo$_{6}$S$_{8}$ and PbMo$_{6}$S$_{8}$. Because of the small sampling depth of the tunneling electrons at higher energies and difficulties with the preparation of ideal tunnel junctions, we obtained indications for the internal phonon modes in PbMo$_{6}$S$_{8}$ in only a few cases. Comparing the tunneling measurements of Cu$_{1.8}$Mo$_{6}$S$_{8}$ with the phonon density of states G(w) determined from neutron scattering we find three low energy peaks at $\sim$3, 4.3 and 6.3 meV instead of one broad maximum at about 6 meV. We attribute this to a structural phase transition involving the copper ions occurring at 268 K.

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